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If you can't write a simple constraint for an access to memory then you should have "memory" on the clobber list. Yes, even if you don't change memory. This situation would arise if you were to add a run-time offset to a pointer and dereference the result. If the compiler has been keeping memory values cached in registers it will have to flush them all before a read or write to an unpredictable location on the chance that one of the cached locations may be the target. If you do change memory and need to use the memory-clobber constraint then you should use "asm volatile" instead of just "asm"; "volatile" keeps optimization from removing the asm code which is important if the code has some crucial side effect that the compiler can't deduce from the constraints. You will also need "volatile" if your assembly code depends on a voltile data source such as an I/O register. Remember that the compiler doesn't analyze the actual instructions in inline assembly code.
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